The present invention relates to a concrete composition, and more particularly to a concrete composition which has high fluidity and filling property and from which a concrete structure having high quality and durability and being excellent in resistance to freezing and thawing can be made without compaction by vibration.
A concrete composition is in general prepared by adding cement and water, and, if necessary, an air entraining agent, a water reducing agent, etc. to aggregate such substances as gravel or sand, and mixing the mixture with stirring. In the case where a concrete structure, in particular, one having a complicated structure, is made of a concrete composition having such composition, it is necessary to carry out sufficient placing and compaction of the concrete composition by using a vibrator or a tamping bar in order to extend the concrete composition to all the corners of the structure. Presently, however, it is difficult to carry out sufficient placing and compaction of a concrete composition due to a marked shortage of experienced workers, elimination or reduction of labor, etc.
The use of concrete composition which has improved fluidity and is prepared by increasing the water content per unit volume of concrete or by adding a superplasticizer or a water reducing agent in order to eliminate the step of placing and compaction to improve execution is increasing. In the case of improving the fluidity of a concrete composition as described above, however, material segregation (separation of aggregate and cement paste and/or setting of the aggregate) easily occurs, so that the quality of the structure made of the concrete composition, such as homogeneity, is impaired. In the concrete composition, bleeding also easily occurs, and bleeding water is collected in the area under the aggregate, reinforcing bars, etc. Water enters from the outside via the traces of the bleeding water and the reinforcing bars rust, so that the durability of the concrete structure is reduced.
Therefore, the development of a concrete composition from which a concrete structure having high quality and durability can be made independent of the technical level of workers and/or any process of making the structure is highly desired, and various concrete compositions having well-balanced fluidity and resistance to material segregation, the placing of which can be carried out without compaction by vibration, have been developed by using a viscosity improver comprising a water-soluble polymer.
One such concrete compositions is a concrete composition developed by the inventors of the present invention (Japanese Laid-Open (KOKAI) Publication 3-45544 (45544/1991)). The concrete composition contains a cellulose or acrylic type of a viscosity improver and a superplasticizer and has a 45 to 80 cm slump flow value, thereby having such a high fluidity and resistance to material segregation as to be capable of sufficiently carrying out close placing without compaction by vibration. Since the viscosity improver may increase the air content in the concrete composition to reduce the strength of the concrete structure due to the foaming property of the above type of viscosity improver, the concrete composition furthermore contains an antifoaming agent for breaking comparatively large bubbles formed with the viscosity improver, and simultaneously an air entraining agent for forming comparatively fine bubbles. As a result, 3 to 6% by volume of air in the form of bubbles which are as fine as possible is introduced to the concrete composition as a whole, the air resistance to freezing and thawing of the hardened concrete composition thereby being improved.
It has now been found that the above concrete composition has the following disadvantages because an antifoaming agent is contained:
As seen from Table 1, which shows the results of the experiment of the above-described concrete composition carried out by the inventors of the present invention, the action of the antifoaming agent continues for some time so that the air content just after preparing the concrete composition and the one in the concrete composition after it is hardened is fairly different. Therefore, it is difficult to adjust the air content to be contained in a hardened concrete composition precisely to the specified air content for ensuring resistance to freezing and thawing, that is, 3 to 6% by volume.
TABLE 1 ______________________________________ Relationship between Air Content in Conventional Concrete Composition and Resistance to Freezing and Thawing Resis- Air Content in Conventional Concrete Composition*.sup.1) tance to (% by volume) Freezing Just After After After After After and Preparation 15 min. 30 min. 50 min. Hardening Thawing ______________________________________ 6.0 4.1 -- -- 3.7 good 7.5 5.4 -- -- 3.2 good 11.0 8.6 6.2 -- 5.4 good 10.5 7.4 6.8 5.8 5.3 good ______________________________________ *.sup.1) Contained Viscosity Improver: methyl cellulose (0.3% by weight, based on water) Contained Antifoaming Agent: a polar organic composition (0.03 to 0.05% b weight, based on water)
As can also be seen from Table 2, which shows the results of the other experiment carried out by the inventors of the present invention, an antifoaming agent breaks even fine bubbles formed by an air entraining agent and has a bad influence on the improvement of resistance to freezing and thawing of a concrete structure.
TABLE 2 ______________________________________ Relationship of Air Content in Concrete Structure after Hardening, Specific Surface of Bubbles and Coefficient of Bubble Distance Air Content in Concrete Specific Coefficient Structure after Surface of Bubble Hardening of Bubbles Distance (% by volume) (cm.sup.2 /cm.sup.3) (.mu.m) ______________________________________ High-fluid 3.7 156 367 Concrete 5.8 164 322 Composition Containing Antifoaming Agent*.sup.2) Conventional 4.0 250 221 Concrete 4.7 259 231 Composition*.sup.3) ______________________________________ *.sup.2) Contained Viscosity Improver: methyl cellulose (0.3% by weight, based on water) Contained Antifoaming Agent: a polar organic composition (0.03 to 0.05% b weight, based on water) *.sup.3) This composition contains neither any viscosity improver nor any antifoaming agent.
That is, from Table 2, it can be found that the concrete structure made of the above concrete composition having high fluidity and containing both a viscosity improver and an antifoaming agent has a smaller specific surface of bubbles and a larger coefficient of bubble distance than those of the concrete structure made of the conventional concrete composition with neither viscosity improver nor antifoaming agent even though these concrete structures have similar air content, that is, the antifoaming agent breaks even the fine bubbles formed by the air entraining agent.
As a result of research taking the above circumstances into consideration, the inventor has found that the above disadvantages can be avoided by using a cellulose type of viscosity improver having a specific property and/or an acrylic type of viscosity improver having a specific property in a specified amount.